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The field of the invention is motor controllers and more specifically control algorithms for use with thyristor based controllers for balancing the positive and negative current half-cycles within a motor phase.
One type of commonly designed induction motor is a three phase motor having three Y-connected stator windings. In this type of motor, each stator winding is connected to an AC voltage source by a separate supply line, the source generating currents therein. Most utilities which supply power to industrial motors supply well balanced purely sinusoidal three phase voltages (and corresponding currents) that have equal amplitudes and periods and are out of phase by exactly 120xc2x0. Nevertheless, for various reasons, switching assemblies and corresponding controllers have been developed to alter the sinusoidal current and voltage waveforms at the point of utilization.
For example, one common three phase switching assembly includes three solid state switching devices, one device for each separate system phase. Each device is positioned in series between the source and the motor phase and can generally be used to control the current passed from the corresponding supply line to the corresponding motor phase. An exemplary switching device may include a pair of separately controllable silicon controlled rectifiers (SCRs) linked together in an inverse parallel relationship so that each SCR is arranged to conduct current in a direction opposite that of the other. Inverse parallel SCRs are commonly referred to as thyristors.
As well known in the art, an SCR does not conduct until after the SCR is triggered (i.e., turned on) and voltage there across is of a polarity that is consistent with the direction in which the SCR conducts. When voltage across an SCR is consistent with the direction in which the SCR conducts, once triggered, the SCR remains in a conductive state until current through the SCR drops to a zero value at which point the SCR turns off. Because the voltage across the SCR is sinusoidal and the current through the SCR is related to (i.e., lags behind) the voltage across the SCR, the SCR turns off within one-half of a line voltage cycle.
According to at least one control scheme, thyristor SCRs are alternately fired to provide alternating positive and negative current half-cycles to a corresponding motor phase. Each pair of consecutive alternating current half-cycles are separated by a xe2x80x9cnotchxe2x80x9d period corresponding to the turn off or shutoff angle of a first SCR in the pair and the turn on or fire angle of the second SCR in the pair. For instance, after a first positive current conducting SCR is fired while the voltage there across is positive, the first SCR remains conducting and current magnitude therethrough rises until the voltage across the SCR reaches a zero value. When the voltage across the first SCR becomes negative, the current magnitude through the first SCR begins to decrease. Eventually, the current through the first SCR drops to a zero value at a first SCR turn off or shutoff angle and the notch period begins.
At the end of the notch period and during the negative half-cycle of the line voltage, the second SCR in the pair is fired, the second SCR begins to conduct and the current magnitude through the second SCR rises until the line voltage again reaches a zero value. When the voltage across the second SCR becomes negative, the current magnitude through the second SCR begins to decrease and eventually drops to a zero value at a second SCR turn off or shutoff angle and a second notch period begins. At the end of the second notch period and during the next positive half-cycle of the line voltage, the first SCR in the pair is again fired, the first SCR begins to conduct and the current magnitude through the first SCR rises until the line voltage again reaches a zero value and the process above is repeated.
Other control schemes call for skipping line voltage cycles to alter motor speed. For instance, according to one scheme, the first SCR in each thyristor is fired during a first line voltage cycle, no SCR is fired during a second voltage cycle, the second SCR in each thyristor is fired during a third line voltage cycle, no SCR is fired during a fourth voltage cycle, the first SCR is again be fired during a fifth line voltage cycle and the pattern is repeated in a continuous fashion. Thus, by controlling the notch periods and, more specifically, the fire times of thyristor SCRs, currents provided to motor phases are controllable.
During a motor starting protocol, after an equipment operator applies a starting signal to the motor controller, the motor controller gradually increases the amount of current applied to the motor by regulating the SCR fire angles. By regulating the fire angles, the controller turns on each thyristor initially for only a portion of each half-cycle of the line voltage for the corresponding motor phase (i.e., the notch periods comprise relatively long portions of each voltage half-cycle). The controller then gradually increases the half-cycle on time of the thyristors (i.e., reduces notch periods), thus gradually increasing stator currents, until the motor is at substantially full speed. This technique reduces the current consumption and torque on the motor during start-up as compared to a hard switching of the full supply line voltage across the motor.
Thus, in thyristor based control systems like the one described above, motor control is premised on a simple algorithm for notch control and more specifically, a simple algorithm for identifying SCR fire times. In essence, according to the simplest fire angle algorithms, the fire angle for one SCR in a thyristor is calculated by adding a desired notch period to a most recent shutoff angle corresponding to the other SCR in the thyristor.
When a simple fire angle algorithm like the one described above is employed, the relationship between the shutoff angle of one SCR in a thyristor pair and fire angle of the other SCR in the pair is wholly a function of the algorithm and does not account for actual motor operating conditions (i.e., fire angle=shutoff angle plus notch period). However, the SCR shutoff angles are directly related to motor operating conditions and are also related to the fire angles at which the SCR is triggered. For instance, all other things being equal, if a fire angle is delayed by 10 degrees the corresponding shutoff angle for the SCR will be expedited by approximately 10 degrees (at least during steady state operation). As another instance, if the power factor PF angle (i.e., the angle corresponding to the delay between the line voltage and the line current) is altered, then the duration between a fire angle and a shutoff angle is also altered. Other changes to the operating conditions that affect shutoff angles are contemplated.
Despite its intuitive form, unfortunately the simple fire angle algorithm described above operates ineffectively under certain circumstances. To this end, there are conditions in which the simple fire angle algorithm causes a system to become xe2x80x9cundesirably stablexe2x80x9d. Here, the phrase xe2x80x9cundesirably stablexe2x80x9d is used to refer to conditions wherein the system is stable despite unbalanced positive and negative current half-cycles within each motor phase (e.g., a phase current may include positive half-cycles having a greater magnitude than negative half-cycles or vice versa). For instance, referring to FIG. 1, a full cycle of exemplary undesirably stable phase voltage and current waveforms 150 and 152 are illustrated where the notch period is 50 degrees. A first shutoff angle occurs at 50 degrees and thus at 100 degrees (i.e., after a 50 degree notch) a first fire angle occurs and current is conducted through a corresponding SCR. The SCR continues to conduct until current therethrough reaches a zero value at shutoff angle 240 degrees which is 60 degrees after the most recent voltage zero crossing at angle 180 degrees. At this point, according to the simple fire time algorithm, the next fire time is calculated to be 290 degrees (i.e., shutoff angle 240 degrees plus the 50 degree notch) and the next shutoff time is 410 degrees which is 50 degrees into the next cycle. Thus, in the illustrated example, the positive half-cycles each have a larger firing duration (i.e., the duration between a fire angle and a shutoff angle) than the negative half-cycles and hence the positive current magnitude is greater than the negative current magnitude despite the fact that the notch widths are the same.
One way to understand this phenomenon is to consider what happens when the first notch illustrated in FIG. 1 is shifted. For instance, assume that the first notch in FIG. 1 (i.e., the notch between 50 and 100 degrees) is shifted to the left 10 degrees while keeping the notch width (e.g., 50 degrees) constant. In this case, the shift first fire angle begins at 40 degrees (i.e., 10 degrees earlier than illustrated) and the notch on the right is shifted further to the right. The end result is that the firing duration beginning at the 90xc2x0 angle (i.e., shutoff angle 40xc2x0 plus the 50xc2x0 notch) becomes wider while the subsequent firing duration that begins at the end of the second width becomes shorter.
The imbalance illustrated in FIG. 1 can be caused by at least two sets of operating circumstances that are relatively common during motor operation. First, shutoff detection errors can result in imbalance. For instance, where a shutoff angle is erroneously detected to have occurred 5 degrees after an actual shutoff angle, the following fire time will be off by 5 degrees. Second, changes in notch size can result in imbalance. Both shutoff angle detection errors and notch size changes are common in thyristor based control schemes and therefore the problem associated with undesirably stable systems must be resolved.
One solution to the imbalance problem has been to add a damping term to the simple fire angle algorithm according to the following equation:
Fc=SOp+Dt+NWxe2x80x83xe2x80x83Eq. 1
Where Fc is the next firing angle, SOp is the previous shut-off angle, Dt is the damping term and NW is the desired notch width. Damping term Dt, in at least one case, has been calculated as a function of the difference between consecutive xe2x80x9clagxe2x80x9d periods where a lag period corresponds to the duration between a shutoff angle and a previous voltage zero crossing. For instance, in FIG. 1, a first lag value would be 50 degrees (i.e., shutoff angle 50 degrees less the zero crossing at 0 degrees) and a second lag value would be 60 degrees (i.e., shutoff angle 240 degrees less zero crossing at 180xc2x0). The damping term Dt for a specific fire angle Fc is calculated as the difference between the two subsequent lags multiplied by a gain term kd according to the following equation:
Dt=(Lag1xe2x88x92Lag2)*kdxe2x80x83xe2x80x83Eq. 2
This modification to the simple fire angle algorithm moves the shutoff angles toward the same positions in each half-cycle thereby tending to balance the positive and negative half-cycles. Unfortunately, control systems that include a damping gain kd requires a settling time in order to achieve a steady state operating condition. The settling time results in a restriction on how fast the notch width can be changed and, because the notch size is a primary control variable in thyristor based control schemes, this restriction undesirably affects overall system response.
Thus, it would be advantageous to have a system that employs a fire angle algorithm that is related to actual operating characteristics of the system and that quickly and accurately identifies fire angles to balance positive and negative half-cycle currents.
It has been recognized that instead of tying fire angles solely to shutoff angles, fire angles can be related to recent system operating characteristics. In this way fire angles are selected as a function of actual motor operating characteristics and thus, changes in system operation are reflected in the fire angles and positive and negative half-cycle balance is attained in a simple, quick and inexpensive manner. More specifically, instead of adding a desired notch duration to a shutoff angle to identify a next fire angle as in the prior art, the present invention identifies an expected peak current angle (referred to herein as a next virtual zero crossing (VZC) angle) for a voltage half-cycle and subtracts a fraction of an ideal firing period therefrom to identify the fire angle for the half-cycle. A peak current magnitude occurs generally at a half way point between a fire angle and a following shutoff angle. The ideal firing period is the period between a fire angle and a consecutive shutoff angle. Thus, where one-half of the ideal firing period is subtracted from a peak current angle, the resulting angle is an optimal fire angle for a cycle. Hence, where the next peak current angle can be identified prior to a fire angle occurring, one-half the firing period can be subtracted from the next peak current angle to identify the optimal next fire angle.
The VZC angle (e.g., peak current angle) for a cycle can be determined in one of two ways. First, where a valid shutoff angle is obtained during a half-cycle (i.e., during an xe2x80x9coccurringxe2x80x9d half-cycle), the shutoff angle and the preceding fire angle can be combined to identify a next VZC angle. For instance, in one embodiment a preceding VZC angle that occurs during a half-cycle immediately preceding the occurring half-cycle can be identified by adding one half the difference between the most recent shutoff angle and the preceding fire angle to the preceding fire angle. Thereafter, 180 degrees is added to the preceding VZC angle to provide the next VZC angle.
Second, where a valid shutoff angle has not been obtained during the occurring half-cycle (i.e., within an expected window), the most recently acquired valid shutoff angle and immediately preceding fire angle can be combined to identify a VZC angle for a previous voltage half-cycle and the difference between the VZC angle for the previous voltage half-cycle and the actual voltage zero crossing angle for the previous voltage half-cycle can be stored to be subsequently used as an estimate of the difference between an actual (e.g., sensed) voltage zero crossing angle and corresponding next VZC angle during the current half-cycle. Here, the difference between a VZC angel and an associated voltage zero crossing angle corresponding to a previous valid shutoff angle is referred to as a zero crossing delta (ZCxcex94) value. After the ZCxcex94 is stored, when a shutoff angle does not occur or is not sensed for some reason during an expected time range, the ZCxcex94 can be added to a sensed voltage zero crossing angle to estimate the next sensed voltage zero crossing angle to estimate the next VZC which is then used to identify the next firing angle.
Thus, it should be appreciated that each of the two methods described above employ data that directly reflects how the motor is instantaneously operating to determine the next fire angle and therefore that the resulting fire angles are selected so that the positive and negative half-cycles are essentially balanced.
Consistent with the above teachings, the invention includes a method for operating a motor controller to cause balanced positive and negative current half-cycles in a motor phase, the motor controller utilizing switching devices to periodically connect a motor phase stator winding to a current source supply line in which each switching device enters a non-conductive state during a notch period of each alternating current half-cycle, each notch period beginning at a shut-off angle when current through the respective switching device becomes zero and ending at a fire angle when the respective switching device is again placed in a conductive state, the controller identifying a desired fire angle, the method comprising the steps of, for an occurring half-cycle, identify a next virtual zero crossing (VZC) angle corresponding to an angle estimate during the current half-cycle at which the magnitude of the fundamental component of the current will begin to decrease, mathematically combining the next VZC angle and the desired notch period to identify the fire angle for the occurring half-cycle and repeating the method for the next half-cycle as the occurring half-cycle.
In at least one embodiment the step of identifying a VZC angle includes the steps of identifying a shut-off angle during the occurring half-cycle and a fire angle preceding the identified shutoff angle and mathematically combining the identified shutoff angle and the identified fire angle. In a more specific embodiment the step of mathematically combining the identified fire angle and the identified shutoff angle includes identifying a fraction of the difference between the identified fire angle and the identified shut-off angle as a most recent firing period fraction and adding the most recent firing period fraction to the identified fire angle to identify a previous VZC angle where the previous VZC angle corresponds to the angle during the previous half-cycle at which the magnitude of the fundamental component of the current began to decrease. In an even more specific embodiment the step of identifying a fraction includes dividing the difference between the identified shut-off angle and the identified fire angle by 2.
In at least one embodiment the step of mathematically combining to determine the next VZC angle further includes adding 180 degrees to the previous VZC angle. In a more specific embodiment the step of determine the next fire angle further includes subtracting the notch period from 180 degrees to identify a firing period, identifying a fraction of the firing period and mathematically combining the firing period fraction next VZC angle to identify the next fire angle. In a further embodiment the step of identifying a fraction includes dividing the firing period by 2.
In one embodiment the step of identifying a fraction includes receiving a notch control value between 0 and 1 and multiplying the firing period by the notch control value. Here, the step of identifying a most recent firing period fraction may include multiplying the difference between the identified fire angle and the identified shut-off angle by the notch control value.
One embodiment further includes the steps of, prior to identifying the fire angle, identifying a shut-off angle window during the occurring half-cycle during which a shut-off angle is expected to occur and determining if a shut-off angle occurs in the angle window, if a shut-off angle occurs in the angle window, performing the process above, else, performing an alternative process to determine the next fire angle.
The step of performing an alternative process may include the steps of identifying the most recent voltage zero crossing angle, identifying a ZCxcex94 value and mathematically combining the most recent voltage zero crossing angle, the ZCxcex94 value and the notch period to determine the next fire angle.
In one embodiment the step of identifying a ZCxcex94 value includes the step of identifying the most recently obtained valid shutoff angle as a valid shutoff angle, the fire angle immediately preceding the valid shutoff angle as a valid fire angle and the voltage zero crossing angle immediately preceding the valid fire angle as a valid voltage zero crossing angle, combining the valid shutoff angle and the fire angle to identify a VZC angle estimate for a corresponding half-cycle, and identifying the difference between the estimated VZC angle and the valid voltage zero crossing angle as the ZCxcex94 value. More specifically, the step of combining the valid shutoff angle and the valid fire angle may include dividing the sum of the valid shutoff angle and the valid fire angle by 2 to identify the VZC angle.
The invention also includes a method for operating a motor controller to cause balanced positive and negative current half-cycles in a motor phase, the motor controller utilizing switching devices to periodically connect a motor phase stator winding to a current source supply line in which each switching device enters a non-conductive state during a notch period of each alternating current half-cycle, each notch period beginning at a shut-off angle when current through the respective switching device becomes zero and ending at a fire angle when the respective switching device is again placed in a conductive state, the controller providing a desired notch period, the method comprising the steps of, for an occurring half-cycle, identifying the most recently obtained valid shut-off angle, identifying the fire angle immediately preceding the identified shut-off angle, mathematically combining the identified fire angle and the identified shut-off angle to identify a next virtual zero crossing (VZC) angle, mathematically combining the next VZC angle and the notch angle to determine a fire angle for the current half-cycle and repeating the method for the next half-cycle as the occurring half-cycle.
In a more specific embodiment the step of mathematically combining to determine the next VZC angle includes performing a first process when the most recently occurring shut-off angle occurs in the occurring half-cycle and includes performing a second process when the most recently occurring shut-off angle occurs in other than the occurring half-cycle. Here, the step of performing the first process may include adding the identified fire angle to a fraction of the difference between the identified fire angle and the identified shut-off angle to provide a preceding VZC angle corresponding to the half-cycle immediately preceding the occurring half-cycle and adding 180 degrees to the preceding VZC angle to identify a next VZC angle. Also, the step of performing the second process may include identifying a most recent voltage zero crossing angle immediately preceding the current half-cycle, identifying a ZCxcex94 value corresponding to the most recently obtained valid shutoff angle, the fire angle immediately preceding the most recently obtained shutoff angle and the voltage zero crossing angle immediately preceding the fire angle and adding the ZCxcex94 value to the current voltage zero crossing angle.
The step of mathematically combining to determine the next fire angle may include subtracting the notch period from 180 degrees to provide a firing period and subtracting a fraction of the firing period from the next VZC angle.
The invention also includes an apparatus for operating a motor controller to cause balanced positive and negative current half-cycles in a motor phase, the motor controller utilizing switching devices to periodically connect a motor phase stator winding to a current source supply line in which each switching device enters a non-conductive state during a notch period of each alternating current half-cycle, each notch period beginning at a shut-off angle when current through the respective switching device becomes zero and ending at a fire angle when the respective switching device is again placed in a conductive state, the controller providing a desired notch period, the apparatus comprising a processor running a pulse sequencing program to perform the steps of, for an occurring half-cycle, identifying the most recently obtained valid shut-off angle, identifying the fire angle immediately preceding the identified shut-off angle, mathematically combining the identified fire angle and the identified shut-off angle to identify a next VZC angle, mathematically combining the next VZC angle and the notch angle to determine a next fire angle for the occurring half-cycle and repeating the steps above for the next occurring half-cycle.
In a more specific embodiment the program causes the processor to perform the step of mathematically combining to determine the VZC angle by performing a first process when the most recently occurring shut-off angle occurs in the occurring half-cycle and includes performing a second process when the most recently occurring shut-off angle occurs in other than the occurring half-cycle. In an even more specific embodiment the program causes the processor to perform the first process by adding the identified fire angle to a fraction of the difference between the identified fire angle and the identified shut-off angle to provide a preceding VZC angle that occurs during a preceding half-cycle and adding 180 degrees to the preceding VZC angle to provide the next VZC angle.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.